Computer storage of data on direct access data storage devices (DASD) has been employed for many years. Initially, all related data were stored on a single device. Sometimes, of course, when the single device became full, a second device received the overflow data. Another approach to disk data storage is the so-called RAID arrangement. Patterson et al in an article "A CASE FOR REDUNDANT ARRAYS OF INEXPENSIVE DISKS (RAID)" in ACM 1988 on pages 109-116 (March 1988) set forth arguments for distributing related data over a plurality of devices. The main theme of the article relates to mean time to failure (MTTF) with some discussion on performance. Four "levels" of RAID are described. A first level is merely two sets of devices, each set containing a mirror image of the data stored in the other device set. A second level adds the Hamming or parity based error detection and correction codings (ECC). In this arrangement, all data stored in all of the individual disks have to be read before a write operation; the entirety of the data has to be rewritten, whether necessary or not for other operations. The article states that this level has limited application. In a third level a so-called "check" device is provided in a group of the devices. Each time data are written, the check device has to be updated. A plurality of devices store respective portions of data and a single device stores parity information (error detection redundancy) derived from corresponding data stored in the plurality of devices. All the plurality of devices are read out in parallel into a controller of some sort. An ECC redundancy is calculated for each transfer for storage on the check disk. A last and fourth level provides for independent reads and writes from and to the devices in a RAID grouping. Additionally there is a check disk. Each data transfer unit is accessed as a single sector. The ECC redundancy information is calculated over a portion of each data transfer unit. All of the devices are identical and each portion of a transfer unit is stored in the respective devices at the same relative address within each device. Accordingly, all portions of the data transfer unit have identical data transfer rates.
Data storing capacity of each disk in a device is determined by the maximal data rate at a radially outwardmost track, the maximum data storage density at a radially inwardmost data storing track, or a combination of the considerations. To increase the data storing capacity of DASD, so-called zoned disks are used. Otteson in U.S. Pat. No. 4,016,603 shows a four-zone disk for use in so-called fixed-block architecture devices (FBA devices). The count-key-data (CKD) format programming support requires addressable tracks to have a constant data storing capacity, such capacity is also referred to as "track length" even though the physical track length varies with radius, the data storing capacity in bytes of all the tracks in a disk is constant. In multi-zoned disks, the track data-storing capacity is constant within each zone but increases in capacity in outer radial zones. It is desired to used CKD formats in multi-zoned disks, particularly in a group of multi-zoned disks which may be located in one or more disk devices. All CKD format "tracks" must have the same data-storing capacity irrespective of which of the zones are involved in storing CKD formatted data. This arrangement must provide short access times to all portions of the CKD track of data.
The quantity of data desired to be stored as one addressable track of data has been continuously increasing. This desire, inter alia, has resulted in the so-called "cylinder mode" of disk data storage area accessing. A cylinder in a data disk device are all of the data tracks located at the same radial position. In a DASD having eight disks, there are 15 surfaces for the data tracks and one servo surface. Of course, known sector servo techniques can be employed so that an eight disk DASD will have 16 data storing surfaces. In any event, the accessing mechanism for such DASD is a comb head assembly which has one head or transducer for each of the 16 surfaces. All tracks being simultaneously scanned are at the same radial position and constitute a cylinder of physical tracks. Switching accessing between physical tracks in any one cylinder is by merely electronically switching different ones of the transducers to read/write circuits of the DASD. The cylinder mode takes advantage of this electronic speed switching to create a single logical track consisting of all physical tracks in a given cylinder of physical tracks. It is also desired to extend the cylinder mode to include physical tracks in more than one cylinder of tracks without sacrificing access performance, i.e. without creating a longer access time to the data. This data storing area access enhancement is termed "extended cylinder mode".